A fault in a power line in an overhead power distribution system can arise from a number of conditions such as lightning striking on or near the line, or animals, fallen trees, wind storms or other conditions damaging the line. Most faults, however, are temporary, that is, the line and its associated circuit are restored after a recloser on the line operates. Temporary faults are therefore corrected without requiring line maintenance. Temporary faults are oftentimes difficult to identify. Utility companies frequently resort to locating or merely confirming the existence of a fault by closing a cutout into the line or circuit suspected of being faulted. If the fuse does not operate or blow, the fault most likely cleared itself. Fuses used as diagnostic tools in this manner, however, all too often blow because the fault did not clear itself.
Fault conditions in an electrical system, whether they occur on the supply or load side, can be hazardous to lineman installing or repairing the system, and to system users. For example, a lineman closing a cutout or fuse connecting a distribution lateral to a main line can be exposed to hot gases, and struck with chards from the fuse, when the fuse explodes due to a fault in the line. Linemen must wear protective equipment and observe many precautions when diagnosing the existence of line faults using this technique. Further, cutouts are subjected to unnecessary wear when they are closed into a faulted line, thereby reducing the life of the cutout. Additional damage to the circuit to which the faulted line is connected can occur from repeated and unnecessary cutout closing into the faulted line. Finally, the expense of such fault detection measures is considerable if a fuse needs to be replaced every time the existence of a fault condition is confirmed.
A need exists for a tool which allows a lineman to check the condition of a line suspected of being faulted before installing a new fuse and without having to first close the cutout. The same tool should also be useful to diagnose systems that are not a part of overhead power distribution systems such as underground power lines and other types of conductors in supply and load side electrical systems.
Existing fault detection systems suffer from a number of disadvantages and drawbacks. One type of existing fault detection system employs Time Domain Relectometry (TDR), or the radar method, to locate faults on underground and overhead lines. Systems employing TDR typically comprise a device for generating a pulse on the line being tested, and TDR equipment to detect and display reflected responses. These systems require a significant amount of complex and expensive equipment to create the pulse and to display the reflected response waveforms. Further, these systems typically require a highly skilled and trained human operator to interpret the displayed, reflected response waveforms to determine the existence and location of a fault.
Some of the difficulties with detecting faults arise not from test modalities such as TDR, but rather from the test line or circuit itself. For example, if a distribution lateral is tested for a fault, and the fault test involves transmitting a test pulse on the lateral, the pulse may encounter a branch, a fault, the end of the line, a transformer or other termination.
Fault detection systems employing the TDR method are not suited for diagnosing faults on lines having branches. As with a fault, a portion of the pulse is generally reflected from the branch toward the pulse source and has a negative amplitude, and the remaining pulse energy is refracted and travels along the multiple pathways extending from the branch. Existing detection systems using TDR cannot distinguish reflected pulses due to a branch from reflected pulses due to a fault. In addition, a pulse travels along a 1 mile line in approximately 5 microseconds. Reflections from a fault or a branch 1 mile away from the pulse source therefore return in approximately 10 microseconds. Further, the pulse of a 0.1 microfarad capacitor discharging into a 400 ohm distribution lateral does not decay for 200 microseconds. Thus, reflected pulses interact with the incident pulse. Interactions between the reflected and incident pulses cannot be effectively deciphered using a detection method such as TDR. Further, the pulse width of the decaying exponential of the discharging capacitor generally cannot be sufficiently shortened without sacrificing capacitor size and, accordingly, the energy needed to provide a large enough pulse to travel along the lateral without being attenuated too much to be useful. Since a large negative reflection is typically viewed as characteristic of a fault in TDR systems, anything that decreases the size of the reflection decreases the effectiveness of the TDR system in detecting a fault. Thus, the above-described effect of branches on a pulse renders TDR an unreliable method for determining fault on lines with branches.
In addition, fault detection systems employing the TDR method generally operate with a low output voltage and therefore cannot detect a fault on a line having an insulated gap. A need exists for a fault detection system that operates at both high and low voltages and is therefore capable of arcing over small insulative gaps in the fault path which might break down once system voltage is restored.
U.S. Pat. No. 5,345,180 discloses another type of fault detection system wherein a line is provided with a wide square-wave pulse of approximately 0.5 milliseconds in duration. The pulse is generated electronically using a gate turn-off (GTO) thryistor and the system voltage. A current transformer measures the resulting current which is then integrated. The integrated current value is compared with a threshold value to determine if a fault exists. The system is disadvantageous because the pulse generation method is complicated, particularly since a pulse of relatively long duration is required. This is because the current characteristic of a capacitor is the most difficult current characteristic to differentiate from that of a short circuit. The differentiation is more pronounced as the duration of the pulse increases. Hence, a pulse of relatively long duration is necessary.
Another method of detecting high impedance faults is presented in an I.E.E.E. paper entitled "Impulsive Response Analysis of a Real Feeder for High Impedance Fault Detection" by P. R. Silva et al. The paper discusses injecting a feeder to be tested with a pulse from the substation. The feeder response must first be measured, stored and analyzed for a feeder operating under normal conditions. The feeder response for a feeder operating under fault conditions is then measured, stored and analyzed in order to be compared with the normal feeder response. The normal and faulted feeder responses are compared in several ways. First, the time domain responses for a normal feeder and the same feeder operating under faulted conditions are compared. Second, the frequency components of the normal and faulted feeder responses, which are calculated using the Fast Fourier Transform, are compared. Finally, the energy content of the normal and faulted feeder responses is compared.
This method suffers from a number of drawbacks. First, the method requires measurement and storage of normal responses or signatures of all circuits being tested. A fault detection system is needed which works on a multitude of different lateral configurations with varying lengths, loads (e.g., transformers), fault locations, and fault resistance levels without first having to normalize each configuration. Second, the three indexes or tests used to compare the responses of a normal operating feeder and the same feeder under fault conditions do not clearly differentiate changes in the feeder responses due to various reflected pulses, which arise from various switch, fault, load and branch conditions of the feeder being tested. Thus, a need exists for a fault detection system which discriminates the effects of various feeder conditions on the feeder response in order to more reliably identify when particular types of faults have occurred.
In view of the foregoing deficiencies in existing fault detection systems, there exists a need for a more effective method of detecting fault, as well as equipment which employs the method.